[kernel-hardening] Re: [PATCH v2] fs: hardlink creation restrictions

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 21 Feb 2012 13:58:00 -0800
Kees Cook <keescook@chromium.org> wrote:

> On systems that have user-writable directories on the same partition
> as system files, a long-standing class of security issues is the
> hardlink-based time-of-check-time-of-use race, most commonly seen in
> world-writable directories like /tmp. The common method of exploitation
> of this flaw is to cross privilege boundaries when following a given
> hardlink (i.e. a root process follows a hardlink created by another
> user). Additionally, an issue exists where users can "pin" a potentially
> vulnerable setuid/setgid file so that an administrator will not actually
> upgrade a system fully.
> 
> The solution is to permit hardlinks to only be created when the user is
> already the existing file's owner, or if they already have read/write
> access to the existing file.
> 
> Many Linux users are surprised when they learn they can link to files
> they have no access to, so this change appears to follow the doctrine
> of "least surprise". Additionally, this change does not violate POSIX,
> which states "the implementation may require that the calling process
> has permission to access the existing file"[1].
> 
> This change is known to break some implementations of the "at" daemon,
> though the version used by Fedora and Ubuntu has been fixed[2] for
> a while. Otherwise, the change has been undisruptive while in use in
> Ubuntu for the last 1.5 years.
> 
> This patch is based on the patch in Openwall and grsecurity.  I have
> added a sysctl to enable the protected behavior, documentation, and an
> audit notification.
> 
> [1] http://pubs.opengroup.org/onlinepubs/9699919799/functions/linkat.html
> [2] http://anonscm.debian.org/gitweb/?p=collab-maint/at.git;a=commitdiff;h=f4114656c3a6c6f6070e315ffdf940a49eda3279
> 

Looks OKish to me.

> --- a/Documentation/sysctl/fs.txt
> +++ b/Documentation/sysctl/fs.txt
> @@ -32,7 +32,8 @@ Currently, these files are in /proc/sys/fs:
>  - nr_open
>  - overflowuid
>  - overflowgid
> -- protected_sticky_symlinks
> +- protected_hardlinks
> +- protected_symlinks

It's silly to add protected_sticky_symlinks and to then rename it in
the very next patch.  I have reworked my copy of the earlier
fs-symlink-restrictions-on-sticky-directories.patch so that it adds
"protected_symlinks".  I then reworked this patch
(fs-hardlink-creation-restrictions.patch) to add protected_hardlinks. 
Nice and simple.

>
> ...
>
> +static inline void
> +audit_log_link_denied(const char *operation, struct path *link)
> +{
> +	struct audit_buffer *ab;
> +
> +	ab = audit_log_start(current->audit_context, GFP_KERNEL, AUDIT_AVC);
> +	audit_log_format(ab, "op=%s action=denied", operation);
> +	audit_log_format(ab, " pid=%d comm=", current->pid);
> +	audit_log_untrustedstring(ab, current->comm);
> +	audit_log_d_path(ab, " path=", link);
> +	audit_log_format(ab, " dev=");
> +	audit_log_untrustedstring(ab, link->dentry->d_inode->i_sb->s_id);
> +	audit_log_format(ab, " ino=%lu", link->dentry->d_inode->i_ino);
> +	audit_log_end(ab);
> +}

This is far too large to inline.  It has two callsites and gcc is
probably smart anough to uninline it for us.  I queued a patch to
remove the `inline'.

>  /**
>   * may_follow_link - Check symlink following for unsafe situations
> - * @dentry: The inode/dentry of the symlink
> - * @nameidata: The path data of the symlink
> + * @link: The path of the symlink
>   *
> - * In the case of the protected_sticky_symlinks sysctl being enabled,
> + * In the case of the sysctl_protected_symlinks sysctl being enabled,
>   * CAP_DAC_OVERRIDE needs to be specifically ignored if the symlink is
>   * in a sticky world-writable directory. This is to protect privileged
>   * processes from failing races against path names that may change out
> @@ -643,19 +660,20 @@ int sysctl_protected_sticky_symlinks __read_mostly =
>   *
>   * Returns 0 if following the symlink is allowed, -ve on error.
>   */
> -static inline int
> -may_follow_link(struct dentry *dentry, struct nameidata *nameidata)
> +static inline int may_follow_link(struct path *link)
>  {
>  	int error = 0;
>  	const struct inode *parent;
>  	const struct inode *inode;
>  	const struct cred *cred;
> +	struct dentry *dentry;
>  
> -	if (!sysctl_protected_sticky_symlinks)
> +	if (!sysctl_protected_symlinks)
>  		return 0;

This is also too large to inline.  It doesn't matter a lot - it's not a
hot path.  I removed this `inline' as well.  gcc will probably inline
it anyway.

If we were really concerned about speed here, we might decide to inline
the test of sysctl_protected_symlinks and to uninline the remainder of
the function.


>  	/* Allowed if owner and follower match. */
>  	cred = current_cred();
> +	dentry = link->dentry;
>  	inode = dentry->d_inode;
>  	if (cred->fsuid == inode->i_uid)
>  		return 0;
>
> ...
>
> +static inline int may_linkat(struct path *link)
> +{
> +	int error = 0;
> +	const struct cred *cred;
> +	struct inode *inode;
> +	int mode;
> +
> +	if (!sysctl_protected_hardlinks)
> +		return 0;
> +
> +	cred = current_cred();
> +	inode = link->dentry->d_inode;
> +	mode = inode->i_mode;
> +
> +	if (cred->fsuid != inode->i_uid &&
> +	    (!S_ISREG(mode) || (mode & S_ISUID) ||
> +	     ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) ||
> +	     (inode_permission(inode, MAY_READ | MAY_WRITE))) &&
> +	    !capable(CAP_FOWNER))
> +		error = -EPERM;

omigod.  Geeze man, whose side are you on?

I suggest something like this:

--- a/fs/namei.c~fs-hardlink-creation-restrictions-fix-fix
+++ a/fs/namei.c
@@ -692,6 +692,23 @@ static inline int may_follow_link(struct
 	return error;
 }
 
+static bool foobar(struct inode *inode, umode_t mode)
+{
+	/* nice comment */
+	if (!S_ISREG(mode))
+		return true;
+	/* nice comment */
+	if (mode & S_ISUID)
+		return true;
+	/* nice comment */
+	if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP))
+		return true;
+	/* nice comment */
+	if (inode_permission(inode, MAY_READ | MAY_WRITE))
+		return true;
+	return false;
+}
+
 /**
  * may_linkat - Check permissions for creating a hardlink
  * @link: the source to hardlink from
@@ -722,11 +739,8 @@ static int may_linkat(struct path *link)
 	inode = link->dentry->d_inode;
 	mode = inode->i_mode;
 
-	if (cred->fsuid != inode->i_uid &&
-	    (!S_ISREG(mode) || (mode & S_ISUID) ||
-	     ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) ||
-	     (inode_permission(inode, MAY_READ | MAY_WRITE))) &&
-	    !capable(CAP_FOWNER))
+	if (cred->fsuid != inode->i_uid && foobar(inode, mode) &&
+			!capable(CAP_FOWNER))
 		error = -EPERM;
 
 	if (error)

Please take a look at that, pick a replacement for foobar, fill in the
nice comments which explain our reasoning, retest it and send it back
at me?

Also please take a look at local variable "mode" and decide if there
was a good reason for it being `int' instead of mode_t?